Aerating assembly for froth flotation cells



July 23, 1968 LOGUE ET AL 3,393,802

AERATING ASSEMBLY FOR FROTH FLOTATION CELLS Original Filed Feb. 5, 19642 Sheets-Sheet 1 A fh C DINVENTOIf/S r W aman, r: Leland H. LogueATTORNEYS July 23, 1968 LOGUE ET AL 3,393,802

AERATING ASSEMBLY FOR FROTH FLOTATION CELLS 5, 1964 2 Sheets-Sheet 2Arthur C. Damon, Jr: 1 1g- BY Leland H. Logue ATTORNEYS United StatesPatent 3,393,802 AERATING ASSEMBLY FOR FROTH FLOTATION CELLS Leland H.Logue, Denver, and Arthur C. Daman, Jr., Englewood, Colo., assignors toDenver Equipment Company, Denver, Colo., a corporation of ColoradoContinuation of application Ser. No. 342,068, Feb. 3, 1964. Thisapplication Dec. 14, 1966, Ser. No. 601,804 Claims. (Cl. 209169)ABSTRACT OF THE DISCLOSURE Froth flotation machine of the hog-troughtype having mechanical aerator and agitator assemblies recirculatingpulp from upper portion of contained pulp body as sole pulp feed toenclosed impellers and flooding impeller intake above elevation of gasinput thereto.

This application is a continuation of applicants application Ser. No.342,068, now abandoned, filed Feb. 3, 1964 for Aerating Assembly forFroth Flotation Cells.

This invention relates to an aerating assembly for froth flotation cellsand is an improvement of the aerating mechanism shown in Logue PatentNo. 2,928,543, dated Mar. 15, 1960, assigned to the assignee of thisapplication.

Froth flotation practices have undergone substantial changes in recentyears, including the treatment of much coarser particles than could beeffectively concentrated by earlier treatment methods. As an example,potash ores are now treated commercially in a size range of minus 6 plus14 mesh. In order to obtain an effective flotation of the coarserparticles in such a pulp, a large volume of aerating gas is introducedunder pressure and is distributed through the pulp body in the cell toelevate the conditioned particles to the surface.

Where the gas is delivered into a covered impeller for mixing with pulpand discharge across the periphery of the impeller, the release of thegas within the impeller enclosure develops a back pressure conditionwhich allows gas to escape through openings in the cover producingerratic and inefficient aeration. Such erratic aeration usually requiresan undue amount of operator regulation and materially reduces flotationefficiency.

It is an object of our invention to provide a simple, durable andefficient aerator assembly for froth flotation cells which distributeslarge volumes of aerating gas in a uniform manner through the pulpcirculated by the assembly.

Another object of this invention is to provide a simple, economical andeflicient aerating assembly which is capable of selective control of theaerating action of a flotation cell with a minimum of operator control.

A further object of our invention is to provide a novel aeratingassembly for flotation cells which has provision for offsetting backpressures in discharging large volumes of aerating gas into the flow ofpulp being circulated by said assembly.

Other objects reside in novel details of construction and novelcombinations and arrangements of parts, all of which will be set forthin the following description.

One advantage of the arrangement of our aerating assembly is that it maybe installed in existing flotation cells as a replacement for otheraerating units, and also may be incorporated in new equipment designedto utilize such an aerator. One of the innovations of the presentinvention is the provision for recirculation of pulp from an upper levelin the cell into and through the impeller enclosure which therebyestablishes a displacement effect which moves previously settled solidsinto suspension with at least a part of the material so elevated beingentrained in the recirculating flow and directed into the impeller whereit is subjected to additional aeration. As a consequence, the particlesadequately conditioned for flotation which are recirculated in thismanner are elevated through the pulp in the impeller discharge and arecarried to the surface as concentrate.

Another innovation is the utilization of a restrictive intake for theimpeller enclosure establishing a gravitational pulp flow creating ahydrostatic seal over the impeller so that the aerating gas dischargedonto the impeller is prevented from developing a back pressure escapeand is discharged across the periphery of the impeller in a uniformmanner effecting substantially uniform distribution throughout the pulpbody.

This control permits gas introduction under relatively high pressure,i.e., pressure in excess of normal for this type of impeller enclosure.However, it should be understood that such high pressure must be lessthan the hydrostatic pressure at the intake opening in order to directthe gas flow into converging relation to the pulp being subjected to themixing action of the impeller.

The essential features of the invention may be incorporated in a varietyof structural embodiments and typical embodiments have been illustratedin the accompanying drawings. The practice of our invention will bedescribed with reference to said drawings. In the drawings, in theseveral views of which like parts bear similar reference numerals,

FIG. 1 is a vertical section of one embodiment of an aerating assemblyaccording to our invention illustrated as installed in a flotation cell;

FIG. 2 is a fragmentary vertical section of another embodiment ofaerating assembly illustrated in relation to the bottom portion of aflotation cell in which it is installed;

FIG. 3 is a vertical section of a third embodiment of aerating assemblyutilizing features of our invention; and

FIG. 4 is a vertical section of still another embodiment utilizingfeatures of our invention and shown as installed in a froth flotationcell.

Referring first to FIG. 1, the flotation cell is shown as having a flatbottom 10, an upright rear wall 11 and a forward upright wall having anupper inclined portion 12 terminating in a froth overflow lip 12x. Whileomitted in the illustration, it will be understood that superstructurewill be mounted at the top of the cell in the usual manner for supportof drive components, control members and the like. A plate or beam 13 isshown in FIG. 1 as representing superstructure or correspondingsupporting members. A shaft 14 is journaled for rotation in a bearingassenmbly 15 or beam 13 and extends downwardly to a point adjacent butspaced from cell bottom 10.

A dished impeller 16 is secured on the lower end of shaft 14 and hasupstanding blades 16x in radial arrangement. A hollow column 17 issupported in a depending position from beam 13 and in enclosing relationto shaft 14. A cover member 18 is secured on the lower end of column 17,and has a central opening 18a and an imperforate top portion 18bextending outwardly therefrom. The column 17 has one or a plurality ofupper openings 19 functioning as a pulp inlet providing a recirculatingflow of pulp onto impeller 16.

A conduit 20 depends from beam 13 in encompassing relation to shaft 14and has a sealed upper end secured on the beam and an open lower end 20xdisposed in close proximity to the top of impeller 16 and at a lowerelevation than the top of :a flanged portion 18x defining the centralopening 18a. The cover member shown in FIG. 1 preferably has a series ofdepending vanes 18:: in radial arrangement which form a partialenclosure through which aerated pulp passes during discharge from theimpeller. The flanged portion 18x is quite thick and in association withthe lower end portion x of conduit 20 defines a restricted orrestrictive passage through which the pulp discharges onto the impeller16. A valve-controlled conduit 21 delivers large volumes of aerating gasinto conduit 20 for discharge through its lower end 20x.

The released gas flows onto the dished surface of impeller 16 onto whichpulp is delivered from the interior of column 17. The impeller isrotated at high speed and the confining relation of the cover and theproximity of the blades to the cover produce an intense beating andmixing action as the aerated pulp is directed to the periphery by thecentrifugal action of the impeller. In such action, back pressuresdevelop which may cause the gas flow to seek escape up column 17. Therestriction of the passage through central opening 18a delays thegravitational flow of pulp from the column intake and maintains a liquidfill in the lower portion of the column which functions as a hydrostaticseal to prevent gas escape into column 17.

It will be apparent that the aerating assembly of FIG. 1 will deliverselected volumes of aerating gas through inlet 21 and conduit 20 ontoimpeller 16. While the impeller is rotating at high speed, pulpcirculating in the cell is entrained through openings 19 in a largervolume than it can discharge through the restricted passage in opening18a and stands to a considerable distance above said opening. This givesan effective head equivalent to the full depth of pulp in the cell.

As a consequence, all of the gas flow passing onto impeller 16 throughend 20x is subjected to the beating and mixing action imparted by blades16x and is discharged in fine bubble formation distributed through thepulp impelled across the periphery of the impeller by its centrifugalaction. The tendency to develop a back pressure condition in suchdischarge is overcome by the depth of the liquid column descendingthrough opening 18a, and the only escape for the gas input is with thedischarge flow passing from the periphery of the impeller.

The elimination of back pressure release permits the operator to set thevalve of inlet 21 to supply any required volume of gas and the operationthereafter requires no additional operator control until a new aeratingrequirement is determined. The mixing of pulp and gas etfects asubstantially uniform dstribution of aerating gas in fine bubbleformation throughout the circulating pulp body in the cell.

The aerated pulp discharge which distributes the gas in fine bubbleformation is a part of the recirculating displacement effect thatprevents solids stratification in the pulp body and fluidizes andelevates previously settled solids which otherwise would remain inertand develop a sanding up effect on the cell bottom. This action permitsselective withdrawal of pulp from an upper portion of the circulatingbody for return through the impeller intake. In such environment andwith proper aeration, the coarse sizes of the pulp, when properlyconditioned, float readily and an eflicient flotation concentration isobtained, even with ore ground in a relatively coarse size range.

The embodiment of our invention shown in FIG. 2 functions similarly tothe assembly shown in FIG. 1, but eliminates the column enclosing theconduit through which the gas is introduced onto the impeller and hasmeans for varying the size of the restricted passage. While details ofthe cell structure are not shown in FIG. 2, they are essentially thesame as in FIG. 1 and comprise a bottom 10, an upright rear wall 11 anda forward wall 12. A shaft 14 carries the impeller 16 disposed in closeproximity to cell bottom 10. An air conduit 20 delivers aerating gasonto impeller 16 in the manner previously described.

The cover 28 in this form is generally similar to cover 18 and has aflanged portion 28x defining a central opening 28a, and the remainingtop surface 28b is imperforate. Connectors 27 secured on conduit 20 areattached to the top portion 23b. Threaded rods 26 actuated from the topof the cell extend through blocks 25 on conduit 20 and carry at theirlower ends an annular wedge-shaped portion 26x which is raised andlowered by the operator to selectively vary the effective size of therestricted passage through which circulating pulp descends onto theimpeller.

This arrangement also provides a hydrostatic seal which effectivelyprevents back pressure leakage and regulates the volume of pulprecirculated from the cell onto the impeller. Some adjustment of theposition of the wedgeshaped member 262: in opening 28a may be requiredin conjunction with the changed setting of the valve regulating gasinput to column 20, but once the desired aeration eifect is attained,the unit will operate for long periods without operator control.

The aerator unit shown in FIG. 3 functions in the same manner as theFIG. 1 embodiment but differs therefrom in structural arrangement. Thesame type of cell is shown having bottom 10, upright rear wall 11, and aforward wall 12 terminating in an overflow lip 12x at its top. A shaft14 is supported for rotation in a bearing assembly 15 on the beam orsuperstructure 13 and an impeller 16 is secured at the lower end of theshaft for rotation therewith. The shaft is enclosed by a conduit 20having an open lower end 20x and a valve-controlled inlet 21 deliversaerating gas into conduit 20' for discharge onto impeller 16.

A tubular member 30, open at both ends seats on and is secured to acover member 31 overhanging the impeller 16. The member 30 may besupported in any suitable manner. As shown, a spider 39 secured onconduit 20 is connected at points of contact with member 30 to providesuch support. Cover member 31 has a central opening 31a bordered by anupstanding flange 31x and its top surface 31b is imperforate to confinematerial moving from opening 31a toward the periphery of the impeller.

In the embodiment shown in FIG. 3, pulp in the cell is recirculatedthrough the opening in the top of member 30 and descends into therestricted passage 31a between flange 31x and conduit end portion 20xthereby maintaining a hydrostatic seal over the impeller. Gas descendingthrough conduit 20 onto impeller 16 under pressure seeks its escape inthe space between the dished surface of impeller 16 and the undersurfaceof cover 31. In such flow, it converges and is mixed with pulp enteringfrom passage 31a and is subjected to an intense beating and mixingaction by the impeller blades before it discharges across the peripheryof the impeller and passes beyond the depending surfaces of cover 31.Back pressure gas release from impeller 16 is prevented by the liquidcolumn above opening 31a, and the entire gas input is discharged in auniform manner.

The form of aerator unit shown in FIG. 4 is quite similar to theembodiment shown in FIG. 2 but omits the adjustment arrangement in thepassage through which pulp descends onto the impeller 16. As shown, thecell has a bottom 10, upright rear wall 11 and forward wall 12terminating in an overflow lip 12x. The shaft 14 on which the impeller16 is disposed adjacent bottom 10 is supported in a bearing assembly 15on beam 13, and is enclosed by a conduit 20 having a valve-controlledgas inlet 21.

The aerating components of this unit include a cover 28, as previouslydescribed, supported from column 20 by a plurality of connectors 37, andthe impeller 16 is secured on the lower end of shaft 14 with the tops ofits blades disposed in closely spaced relation to the undersurface ofcover 28. Circulating pulp of the cell is drawn into the impellerenclosure through the restricted opening 28a and functions as ahydraulic seal to prevent escape of gas if back pressures develop.

Consequently, the pressure discharge of gas from the open end 20x ofcolumn 20 is confined within the impeller enclosure and the gas issubjected to the beating and mixing action of the blades in itsprogressive movement to the point of discharge at the periphery of theimpeller.

It will be apparent from the foregoing description of the severalembodiments shown in the drawings that the opening in the cover throughwhich recirculating pulp descends onto the impeller is restrictedsufliciently to imsure maintenance of a substantial head of liquid overthe opening and under hydrostatic pressure to effectively prevent anygas release through said opening under whatever back pressures maydevelop in the impeller enclosure.

This control feature permits introduction of large volumes of gas underpressure which gas is effectively dispersed through the circulating pulpand provides an intense aeration which after discharge from the impellerraises the coarser sizes of conditioned particles to the surface wherethey are collected and discharged as concentrate.

Another feature common to the several structural embodiments describedhereinbefore is that the pulp-conducting means by which recirculatedpulp from the cell is directed through the cover opening or passage hasan intake capacity exceeding the discharge capacity of the coveropening. This condition is due to the size or shape of theflow-directing surfaces and the 'volume of pulp available at the top ofthe opening which is maintained under substantial hydrostatic pressure.Consequently, the restriction of the passage in its various formsprovides an effective seal against any back pressure tendency and forcesthe discharged gas to seek its escape across the periphery of theimpeller.

In the forms shown in FIGS. 2 and 4, the upstanding flange bordering thecentral opening of the cover functions as the pulp conducting means, andis shaped to provide a greater intake capacity at its top than thedischarge capacity of the lower portion of the opening. If desired, theflanged portion may be extended upwardly to a higher level to returnpulp from an upper portion of the pulp body to the intake flow. In theother forms in which a hollow member conducts recirculating pulp to thecover opening, the intake capacity of such member exceeds its dischargecapacity and produces a corresponding effect.

In all the forms described, the tendency of solids in the pulp tostratify within the pulp body is effectively offset by the recirculatingaction which returns a sufficient quantity of dilute pulp containingsolids in finer sizes in the flow delivered onto the impeller todisplace and fluidize settled solids on the bottom of the cell beyondthe impeller. This displacement eflect maintains solids in all sizes insuspension, thereby permitting the flotation action to functioncontinuously under optimum conditions.

We claim:

1. In a froth flotation cell having a froth overflow in its upperportion and an aerating assembly for circulating and aerating pulp inthe lower portion of the cell, said assembly including 'a verticalrotary shaft, an impeller mounted on the lower end of the shaft forconjoint rotation therewith, a stationary cover member secured inclosely spaced overhanging relation to the impeller and having a centralintake opening, a stationary hollow conduit encompassing and spaced fromsaid shaft and having a bottom discharge outlet at the central intakeopening of the cover member, said conduit being sealed against admissionof pulp throughout its length and having an inlet for aerating gas,means for delivering an aerating gas under pressure through the conduitinlet for discharge onto the impeller, said conduit being of lesserdiameter than the cover intake opening and defining with the sides ofsaid opening a restricted passage through which pulp descends onto theimpeller, pulp-recirculating means exteriorly of the hollow conduithaving a discharge passage into the restricted passage and having asubmerged pulp intake passage in an upper portion of the cell, and thesides of the cover intake opening extending to a plane above thedischarge outlet of the hollow conduit so as to confine pulp descendingthrough the restricted passage in gas sealing relation, and means forvarying the effective volume of said restricted passage inclusive ofvarying means supported in spaced relationship between said impeller andsaid cover member, said varying means being vertically adjustable.

2. In a froth flotation cell having a froth overflow in its upperportion and an aerating assembly for circulating and aerating pulp inthe lower portion of the cell, said assembly including a vertical rotaryshaft, an impeller mounted on the lower end of the shaft for conjointrotation therewith, a stationary cover member secured in closely spacedoverhanging relation to the impeller and having a central intakeopening, a stationary hollow conduit encompassing and spaced from saidshaft and having a bottom discharge outlet at the central intake openingof the cover member, said conduit being sealed against admission of pulpthroughout its length and having an inlet for aerating gas, means fordelivering an aerating gas under pressure through the conduit inlet fordischarge onto the impeller, said conduit being of lesser diameter thanthe cover intake opening and defining with the sides of said opening arestricted passage through which pulp descends onto the impeller,pulp-recirculating means including a hollow imperforate column member insealed connection at its lower end with the cover member and extendingupwardly into the pulp body a substantial distance above the covermember in concentric relation with said hollow conduit, said columnmember 'hav ing a submerged pulp intake opening at the upper portion ofthe pulp body providing the sole pulp intake for the cell and enclosingthe intake opening of the cover member at its lower end to provide adischarge passage into said restricted passage, and the sides of thecover intake opening extending to a plane above the discharge outlet ofthe hollow conduit so as to confine pulp descending through therestricted passage in gas sealing relation.

3. In a froth flotation cell having a froth overflow in its upperportion and an aerating assembly for circulating and aerating pulp inthe lower portion of the cell, said assembly including a vertical rotaryshaft, an impeller mounted on the lower end of the shaft for conjointrotation therewith, a stationary cover member secured in closely spacedoverhanging relation to the impeller and having a central intakeopening, a stationary hollow conduit encompassing and spaced from saidshaft and having a bottom discharge outlet at the central intake openingof the cover member, said conduit being sealed against admission of pulpthroughout its length and having an inlet for aerating gas, means fordelivering an aerating gas under pressure through the conduit inlet fordischarge onto the impeller, said conduit being of lesser diameter thanthe cover intake opening and defining with the sides of said opening arestricted passage through which pulp descends onto the impeller, ahollow imperforate column member in sealed connection at its lower endwith the cover member and extending upwardly into the pulp body asubstantial distance above the cover member, said column member having asubmerged pulp intake opening at the upper portion of the pulp bodyproviding the sole pulp intake for the cell and enclosing the intakeopening of the cover member at its lower end to provide a dischargepassage into said restricted passage, and the sides of the cover intakeopening extending to a plane above the discharge outlet of the hollowconduit so as to confine recirculating pulp descending into and throughthe restricted passage in gas sealing relation.

4. In a froth flotation machine having a froth overflow in its upperportion and an aerating assembly for circulating and aerating pulp inthe lower portion of the machine, said assembly including a verticalrotary shaft, an impeller mounted at the lower end of the shaft forconjoint rotation therewith, a stationary cover member secured inclosely spaced overhanging relation to the impeller and having a centralintake opening, a stationary hollow conduit encompassing and spaced fromsaid shaft and having a bottom discharge outlet at the central intakeopening of the cover member, said conduit being sealed against admissionof pulp throughout its length and having an inlet for aerating gas,means for delivering an aerating gas through the conduit inlet fordischarge onto the impeller, said conduit being of lesser diameter thanthe cover intake opening and defining wtih the sides of said opening arestricted passage through which pulp descends onto the impeller, pulprecirculating means exteriorly of the hollow conduit, inclusive of anupstanding portion having an upper pulp intake opening at an elevationabove the bottom outlet of the conduit, said pulp intake opening beingsubmerged in the pulp body directly above the impeller and providing thesole pulp intake for the impeller, and the intake capacity of the intakeopening of said pulp recirculating means being greater than thedischarge capacity of said restricted passage thereby establishing acontinuous flooding effect over the intake opening of the cover memberand above the bottom discharge outlet of the gas supply conduit fordelivering mixed flows of gas and recirculated pulp directly downwardinto the impeller enclosure.

5. In a froth flotation machine having a froth overflow in its upperportion and an aerating assembly for circulating and aerating pulp inthe lower portion of the machine, said assembly including a verticalrotary shaft, an impeller mounted at the lower end of the shaft forconjoint rotation therewith, a stationary cover member secured inclosely spaced overhanging relation to the impeller and having a centralintake opening, a hollow conduit encompassing and spaced from said shaftand having a bottom discharge outlet at the central intake opening ofthe cover member, said conduit being sealed against admission of pulpthroughout its length and having an inlet for aerating gas, means fordelivering an aerating gas through the conduit inlet for discharge ontothe impeller, said conduit being of lesser diameter than the coverintake opening and defining with the sides of said opening a restrictedpassage through which pulp descends onto the impeller, said cover memberhaving an associated upstanding portion adjoining its central intakeopening inclusive of an upper portion intake opening at an elevationabove the bottom of the outlet of the conduit, and the intake capacityof the intake opening of said pulp recirculating means being greaterthan the discharge capacity of said restricted passage therebyestablishing a continuous flooding effect over the intake opening of thecover member and above the bottom discharge outlet of the gas supplyconduit for delivering mixed flows of gas and recirculated pulp directlydownward into the impeller enclosure.

6. In a froth flotation cell having a froth overflow in its upperportion and an aerating assembly for circulating and aerating pulp inthe lower portion of the cell, said assembly including a vertical rotaryshaft, an impeller mounted at the lower end of the shaft for conjointrotation therewith, a stationary cover member secured in closely spacedoverhanging relation to the impeller and having a central intakeopening, a stationary hollow conduit encompassing and spaced from saidshaft and having a bottom discharge outlet at the central intake openingof the cover member, said conduit being sealed against admission of pulpthroughout its length and having an inlet for aerating gas, means fordelivering an aerating gas through the conduit inlet for discharge ontothe impeller, said condit being of lesser diameter than the cover intakeopening and defining with the sides of said opening a restricted passagethrough which pulp descends onto the impeller, pulp-recirculating meansexteriorly of the ho]- low conduit including a hollow imperforate columnmember in sealed connection at its lower end with the cover member andextending upwardly into the pulp body a substantial distance above thecover member, said column member having an upper pulp intake opening atan elevation above the bottom of the outlet of the conduit, said pulpintake opening being submerged in an upper portion of the pulp bodydirectly above the impeller and providing the sole pulp intake for theimpeller, and the intake capacity of the intake opening of said pulprecirculating means being greater than the discharge capacity of saidrestricted passage thereby establishing a continuous flooding effectover the intake opening of the cover member and above the bottomdischarge outlet of the gas supply conduit for delivering mixed flows ofgas and recirculated pulp directly downward into the impeller enclosure.

7. A cell as defined in claim 6, in which the means for deliveringaerating gas through the conduit inlet is an opening to atmosphere abovethe pulp level in the-cell.

8. In a froth flotation cell having a froth overflow in its upperportion and an aerating assembly for circulating and aerating pulp inthe lower portion of the cell, said assembly including a vertical rotaryshaft, an impeller mounted on the lower end of the shaft for conjointrotation therewith, a stationary cover member secured in closely spacedoverhanging relation to the impeller and having a central intakeopening, a stationary hollow conduit encompassing and spaced from saidshaft and having a bottom discharge outlet within the central intakeopening of the cover member and below the top of said intake opening,said conduit being sealed against admission of pulp throughout itslength and having an inlet for aerating gas, means for delivering anaerating gas under pressure through the conduit inlet for discharge ontothe impeller, said conduit being of lesser diameter than the coverintake opening and defining with the sides of said opening a restrictedpassage through which pulp descends onto the impeller,pulp-recirculating means inclusive of a tubular member in sealedconnection at its lower end with the cover member and extending upwardlyinto the pulp body a substantial distance above the cover member, saidtubular member having an upper pulp intake opening at an elevation abovethe bottom outlet of the conduit, said pulp intake opening beingsubmerged in the upper portion of the pulp body directly above theimpeller and providing the sole pulp intake for the impeller, thetubular body between the intake opening and the sealed connection beingimperforate and enclosing the restricted passage, and the intakecapacity of the intake opening of said pulp recirculating means beinggreater than the discharge capacity of said restricted passage therebyestablishing a continuous flooding effect over the intake opening of thecover member and above the bottom discharge outlet of the gas supplyconduit for delivering rnixed flows of gas and recirculated pulpdirectly downward into the impeller enclosure.

9. A cell as defined in claim 8, in which the hollow conduit, thetubular member and the central intake opening are in concentricarrangement.

10. In a froth flotation cell having a froth overflow in its upperportion and an aerating assembly for circulating and aerating pulp inthe lower portion of the cell, said assembly including a vertical rotaryshaft, an impeller mounted on the lower end of the shaft for conjointrotation therewith, a stationary cover member secured in closely spacedoverhanging relation to the impeller and having a central intakeopening, a stationary hollow conduit encompassing and spaced from saidshaft and having a bottom discharge outlet at the central intake openingof the cover member, said conduit being sealed against admission of pulpthroughout its length and having an inlet for aerating gas, means fordelivering an aerating gas under pressure through the conduit inlet fordischarge onto the impeller, said conduit being of lesser diameter thanthe cover intake opening and defining with the sides of said opening arestricted passage through which pulp descends onto the impeller,pulp-recirculating means exteriorly of the hollow conduit including ahollow irnperforate column member in sealed connection at its lower endwith the cover member and extending upwardly into the pulp body asubstantial distance above the cover member, said column member having asubmerged pulp intake opening at the upper portion of the pulp bodyproviding the sole pulp intake for the cell and enclosing the intakeopening of the cover member at its lower end to provide a dischargepassage into said restricted passage, and the intake capacity of theintake opening of said pulp recirculating means being greater than thedischarge capacity of said restricted passage thereby establishing acontinuous flooding effect over the intake opening of the cover memberand above the bottom discharge outlet of the gas supply conduit fordelivering mixed flows of gas and recirculated pulp directly downwardinto the impeller enclosure.

References Cited UNITED STATES PATENTS HARRY B. THORNTON, PrimaryExaminer.

10 ROBERT HALPER, Assistant Examiner.

